The present disclosure relates generally to proximity detection systems and, more particularly, to a method and apparatus for detecting a change in capacitance of a capacitive proximity sensor.
Various systems have been devised for detecting obstacles in the path of a moveable panel such as an automotive power window, power sliding door or power hinged door. When an obstacle is detected, forward movement (e.g., closing) of the panel is interrupted and, optionally, the movement of the panel may be thereafter reversed (e.g., opened). These detection systems may generally be characterized as either xe2x80x9ccontactingxe2x80x9d or xe2x80x9cnon-contactingxe2x80x9d. In a contacting system, an obstacle is detected only after some form of physical contact occurs between the panel and the obstacle, and may include devices such as pneumatic/pressure sensitive strips, or possibly sensors responsive to changes in mechanical or electrical loading in the apparatus that moves the panel.
On the other hand, in a non-contacting system, an obstacle is detected before actual contact occurs. One specific type of non-contacting obstacle detection system employs the use of a capacitive element(s) as a proximity sensor(s). Capacitive proximity sensors may include one or more electrical conductors formed along the leading edge of a moveable panel, as well as a capacitance sensitive circuit (e.g., a bridge circuit or an oscillator) coupled to the conductor(s). An obstacle (e.g., a human hand) in proximity to the conductor(s) changes the capacitance of the sensor, which change is thereafter detected by the capacitive sensitive circuit.
Unfortunately, certain difficulties are inherent in creating a sensitive capacitive proximity system that can distinguish between changes in environmental conditions and an actual foreign object. A sufficiently sensitive capacitive proximity detection system should be responsive to relatively small changes in sensor capacitance. However, certain high frequency components used in conjunction with a sensitive device can result in unwanted radiated emissions to neighboring electronic components. In addition, any control module used for the processing of a detected capacitance change will generally be provided with a lower frequency integrated circuit(s), thus presenting a compatibility problem between the higher frequency sensor signal and the integrated circuit.
The above discussed and other drawbacks and deficiencies of the prior art are overcome or alleviated by a method for detecting a change in capacitance of a capacitive sensing element having a nominal capacitance value. In an exemplary embodiment, the method includes coupling the sensing element to a first oscillator, the first oscillator generating a first frequency dependent upon the capacitance value of the sensing element. The first frequency is compared to a reference frequency generated by a second oscillator. The change in capacitance from the nominal capacitance value is detected if the first frequency differs from said reference frequency by a determined frequency value.
In a preferred embodiment, the first oscillator and the second oscillator are configured such that the first frequency and the reference frequency are offset from one another by a fixed offset value when the capacitance of the capacitive sensing element is equal to the nominal capacitance value. In addition, the first frequency and the reference frequency are inputted into a mixer. An output of the mixer is then inputted into a low pass filter, wherein an output of the low pass filter is an intermediate frequency (IF) that corresponds to the difference between the first frequency and the reference frequency. The intermediate frequency (IF) is preferably in the range of about 0 to about 15 MHz, while the first frequency and reference frequency are at least 900 MHz.
In still a further embodiment, the intermediate frequency is transmitted to a control module where the intermediate frequency is then compared to the fixed offset value. Thereby, an increase in capacitance from the nominal capacitance value is detected if the intermediate frequency exceeds the fixed offset value.